Soleniods are traditionally used to actuate mechanisms by the application of a voltage to an electromagnetic coil. Solenoids are expensive and require considerable design effort to ensure that the mechanical load requirements are consistent with the available force profile of the solenoid. This can be particularly challenging since the solenoid provides less force near the beginning of its stroke and provides exponentially more force as the stroke reaches the end of its travel. Solenoids suffer reliability problems because magnetic flux must bridge the plunger's sliding bearing and residual magnetic spacer of close tolerance must prevent the plunger from magnetically sticking to the pole face. If either of these design parameters becomes too marginal, the solenoid performance is radically altered.
Linear actuators have been designed where a motor drives a threaded shaft and a corresponding threadedly coupled nut. The nut translates laterally when prevented from rotating by a guiding surface. The motor may be driven in one direction to emulate the drive stroke of a solenoid and driven in the other direction to return the nut and an attached actuator means to a home position. To define the stroke of the motor driven linear actuator, axial stops have been used which may generate thrust loads in the extended or retracted position. To eliminate driving the motor in both directions, a torsion spring has been used with the appropriate thread design to enable the torsion spring to rotate the shaft and translate the nut to a home position. Axial loads may cause motor damage or the threads to bind and thrust return springs, besides causing axial loads, put severe restrictions on the design of the threads to allow a non powered return of an extended actuator.
Co-pending patent application Ser. No. 09/829,888 sets forth, in part: an electrically driven linear actuator device that has neither a sliding bearing nor a requirement for a magnetic residual; a linear actuator that eliminates thrust loads on a drive motor which may otherwise reduce motor life; and a linear actuator device that generates force relatively independent of travel position. The linear actuator is, in part, a low-cost direct current (DC) motor adapted with a threaded shaft mounted into a reference frame which keeps the DC motor case and thus the stator fixed while the threaded shaft rotates.
It is foreseeable that the linear actuator of co-pending patent application Ser. No. 09/829,888 will have an wide variety of applications and be used in demanding environments where operational longevity and reliability may be critical.